A PROCESS OF UNDERFILLING AN INTEGRATED CIRCUIT

Abstract

A partial gel step in the underfilling of an integrated circuit that is mounted to a substrate. The process involves dispensing a first underfill material and then heating the underfill material to a partial gel state. The partial gel step may reduce void formation and improve adhesion performance during moisture loading.

Full Text

FORM 2
THE PATENTS ACT, 1970
[39 OF 1970]
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
[See Section 10; rule 13]
"A PROCESS OF UNDERFILLING AN INTEGRATED CIRCUIT"
INTEL CORPORATION, a Delaware corporation, of 2200 Mission College Boulevard, Santa Clara, California 95052, United Sates of America,
The following specification particularly describes the nature of the invention and the manner in which it is to be performed:-

The present invention relates to a process of underfilling an integrated circuit during assembly in an integrated circuit package.
1. FIELD OF THE INVENTION
The present invention relates to an integrated circuit package.
2. BACKGROUND INFORMATION
Integrated circuits are typically assembled into a package that is soldered to a printed circuit board. Figure 1 shows a type of integrated circuit package that is commonly referred to as flip chip or C4 package. The integrated circuit 1 contains a number of solder bumps 2 that are soldered to a top surface of a substrate 3.
The substrate 3 is typically constructed from a composite material which has a coefficient of thermal expansion that is different than the coefficient of thermal expansion for the integrated circuit. Any variation in the temperature of the package may cause a resultant differential expansion between the integrated circuit 1 and the substrate 3. The differential expansion may induce stresses that can crack the

solder bumps 2. The solder bumps 2 carry-electrical current between the integrated circuit
1 and the substrate 3 so that any crack in the
bumps 2 may affect the operation of the circuit 1.
The package may include an underfill material 4 that is located between the integrated circuit 1 and the substrate 3. The underfill material 4 is typically an epoxy which strengthens the solder joint reliability and the thermo-mechanical moisture stability of the IC package.
The package may have hundreds of solder bumps
2 arranged in a two dimensional array across the
bottom of the integrated circuit 1. The epoxy 4
is typically applied to the solder bump interface
by dispensing a single line of uncured epoxy
material along one side of the integrated circuit.
The epoxy then flows between the solder bumps.
The epoxy 4 must be dispensed in a manner that
covers all of the solder bumps 2.
It is desirable to dispense the epoxy 4 at only one side of the integrated circuit to insure that air voids are not formed in the underfill. Air voids weaken the structural integrity of the integrated circuit/substrate interface. Additionally, the underfill material 4 must have good adhesion strength with both the substrate 3 and the integrated circuit 1 to prevent delamination during thermal and moisture loading. The epoxy 4 must therefore be a material which is provided in a state that can flow under the entire integrated circuit/substrate interface while having good adhesion properties.

The substrate 3 is typically constructed from a ceramic material. Ceramic materials are relatively expensive to produce in mass quantities. It would therefore be desirable to provide an organic substrate for a C4 package. Organic substrates tend to absorb moisture which may be released during the underfill process. The release of moisture during the underfill process may create voids in the underfill material. Organic substrates also tend to have a higher coefficient of thermal expansion compared to ceramic substrates that may result in higher stresses in the die, underfill and solder bumps. The higher stresses in the epoxy may lead to cracks during thermal loading which propagate into the substrate and cause the package to fail by breaking metal traces. The higher stresses may also lead to die failure during thermal loading and increase the sensitivity to air and moisture voiding. The bumps may extrude into the voids during thermal loading, particularly for packages with a relatively high bump density. It would be desirable to provide a C4 package that utilizes an organic substrate.
SUMMARY OF THE INVENTION
One embodiment of the present invention is an integrated circuit package which may include an integrated circuit that is mounted to a substrate. The package may include an underfill material that

is attached to the integrated circuit and the substrate and a fillet which seals the underfill material.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a side view of an integrated circuit package of the prior art;
Figure 2 is a top view of an embodiment of an integrated circuit package of the present invention;
Figure 3 is an enlarged side view of the integrated circuit package;
Figure 4 is a schematic showing a process for assembling the integrated circuit package.
DETAILED DESCRIPTION OF THE INVENTION
Referring to the drawings more particularly by reference numbers, Figures 2 and 3 show an embodiment of an integrated circuit package 10 of the present invention. The package 10 may include a substrate 12 which has a first surface 14 and a second opposite surface 16. An integrated circuit 18 may be attached to the first surface 14 of the substrate 12 by a plurality of solder bumps 20. The solder bumps 20 may be arranged in a two-dimensional array across the integrated circuit 18. The solder bumps 20 may be attached to the integrated circuit 18 and to the substrate 12 with a process commonly referred to as controlled collapse chip connection (C4).

The solder bumps 20 may carry electrical current between the integrated circuit 18 and the substrate 12. In one embodiment the substrate 12 may include an organic dielectric material. The package 10 may include a plurality of solder balls 22 that are attached to the second surface 16 of the substrate 12. The solder balls 22 can be reflowed to attach the package 10 to a printed circuit board (not shown).
The substrate 12 may contain routing traces, power/ground planes, vias, etc. which electrically connect the solder bumps 20 on the first surface 14 to the solder balls 22 on the second surface 16. The integrated circuit 18 may be encapsulated by an encapsulant (not shown). Additionally, the package 10 may incorporate a thermal element (not shown) such as a heat slug or a heat sink to remove heat generated by the integrated circuit 18.
The package 10 may include a first underfill material 24 that is attached to the integrated circuit 18 and the substrate 12. The package 10 may also include a second underfill material 26 which is attached to the substrate 12 and the integrated circuit 18. The second underfill material 26 may form a circumferentic fillet that surrounds and seals the edges of the IC and the first underfill material 24. The sealing function of the second material 26 may inhibit moisture migration, cracking of the integrated circuit and cracking of the first underfill material.

The first underfill material 24 may be an epoxy produced by Shin-Itsu of Japan under the product designation Semicoat 5230-JP. The Semicoat 5230-JP material provides favorable flow and adhesion properties. The second underfill material 26 may be an anhydride epoxy produced by Shin-Itsu under the product designation Semicoat 122X. The Semicoat 122X material has lower adhesion properties than the Semicoat 5230-JP material, but much better fracture/crack resistance.
Figure 4 shows a process for assembling the package 10. The substrate 12 may be initially baked in an oven 2 8 to remove moisture from the substrate material. The substrate 12 is preferably baked at a temperature greater than the process temperatures of the remaining underfill process steps to insure that moisture is not released from the substrate 12 in the subsequent steps. 3y way of example, the substrate 12 may be baked at 163 degrees centigrade (°C).
After the baking process, the integrated circuit 18 may be mounted to the substrate 12, The integrated circuit 18 is typically mounted by reflowing the solder bumps 20,
The first underfill material 24 may be dispensed onto the substrate 12 along one side of the integrated circuit 18 at a first dispensing station 30. The first underfill material 24 may flow between the integrated circuit 18 and the substrate 12 under a wicking action. By way of example, the first underfill material 24 may be

dispensed at a temperature between 110 to 120°C. There may be a series of dispensing steps to fully fill the space between the integrated circuit 18 and the substrate 12.
The package 10 may be moved through an oven 32 to complete a flow out and partial gel of the first underfill material 24. By way of example, the underfill material 24 may be heated to a temperature of 120-145°C in the oven 32 to partially gel the underfill material 24. Partial gelling may reduce void formation and improve the adhesion between the integrated circuit 18 and the underfill material 24. The improvement in adhesion may decrease moisture migration and delamination between underfill material 24 and the IC 18 as well as delamination between underfill material 24 and the substrate 12 . The reduction in void formation may decrease the likelihood of bump extrusion during thermal loading. The package may be continuously moved through the oven 32 which heats the underfill material during the wicking process. Continuously moving the substrate 12 during the wicking process decreases the time required to underfill the integrated circuit and thus reduces the cost of producing the package. The substrate 12 can be moved between stations 3 0 and 34 and through the oven 32 on a conveyer (not shown).
The- second underfill material 26 may be dispensed onto the substrate 12 along all four sides of the integrated circuit 18 at a second dispensing station 34. The second material 26 may

dispensed in a manner which creates a fillet that encloses and seals the first material 24. By way of example, the second underfill material 26 may be dispensed at a temperature of approximately 80 to 120°C.
The first 24 and second 26 underfill materials may be cured into a hardened state. The materials may be cured at a temperature of approximately 150 °C. After the underfill materials 24 and 26 are cured, solder balls 22 may be attached to the second surface 16 of the substrate 12.
While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art.

WE CLAIM:
1. A process of underfilling an integrated circuit during assembly in an
integrated circuit package, the process comprising:
dispensing a first underfill material heated to. a first temperature between the integrated circuit and a substrate; and after dispensing the first underfill material, heating the first underfill material to a second temperature greater than the first temperature to achieve a partial gel state.
2. The process as claimed in claim 1, comprising:
dispensing a second underfill material, the second underfill material attaching to the integrated circuit and the substrate.
3. The process as claimed in claim 2, wherein the second underfill material is dispensed in a pattern which surrounds the first underfill material.
4. The process as claimed in claim 2, wherein the first underfill material is an epoxy and the second underfill material is an anhydride epoxy.
5. The process as claimed in claim 2, wherein the second underfill material is heated to a fourth temperature.
6. The process as claimed in claim 2, comprising:

after dispensing the first underfill material heated to the first temperature, after heating the first underfill material to the second temperature to achieve a partial gel state, and after dispensing the second underfill material,
heating the first underfill material and the second underfill material to a fifth temperature greater than the first and second temperatures to cure the first underfill material and the second underfill material.
7. The process as clairned in claim 1, wherein the first underfill material flows between the integrated circuit and the substrate.
8. The process as clairned in claim 1, comprising:
prior to dispensing the first underfill material heated to the first temperature between the integrated circuit and the susbtrate, heating the substrate to a third temperature greater than first and second temperatures.
9. The process as claimed in claim 8, wherein heating the substrate to
the third temperature greater than the first and second temperatures
removes moisture from the substrate material and avoids moisture
release during the dispensing of the first underfill material at the first
temperature and the heating of the first underfill material to the
second temperature to achieve the partial gel state.

10. The process as claimed in claim 8, wherein the first temperature is in a first temperature range, the second temperature is in a second temperature range greater than the first temperature range, and the third temperature is in a third temperature range greater than the first and second temperature ranges.
11. The process as claimed in claim 1, comprising mounting the integrated circuit to the substrate with a solder bump.
12. The process as claimed in claim 11, comprising attaching a solder ball to the substrate.
13. The process as claimed in claim 1, wherein the dispensing of the first underfill material heated to the first temperature includes,
placing the integrated circuit and the substrate in an oven, and continuously moving the integrated circuit and the substrate to decrease a time for the first underfill material to flow therebetween under a wicking action.
14. The process as claimed in claim 1, wherein the first underfill material
is heated to the first temperature so as to flow between the integrated
circuit and the substrate under a wicking action, and
after dispensing, the first underfill material is heated to the second temperature to achieve the partial gel state so as to reduce void

formation and improve adhesion between the integrated circuit and the underfill material.
15. The process as claimed in claim 1, wherein the integrated circuit package is a controlled collapse chip connection (C4) integrated circuit package.
16. The process as claimed in claim 1, comprising:
prior to the dispensing and the heating of the first underfill material, baking the substrate at a third temperature greater than the first temperature and the second temperature; and mounting the integrated circuit to the substrate.
17. The process as claimed in claim 16, wherein the mounting of the integrated circuit to the substrate uses one or more solder bumps.
18. The process as claimed in claim 17, comprising attaching one or more solder balls to the substrate.
19. The process as claimed in claim 18, wherein the integrated circuit package is a controlled collapse chip connection (C4) integrated circuit package.
20. The process as claimed in claim 16, wherein baking the substrate at the third temperature greater than the first temperature and the

second temperature removes moisture from, the substrate material
and avoids moisture release during the dispensing of the first underfill
material heated to the first temperature and the heating of said first
underfill material to the second temperature to partially gel said first
underfill material.
Dated this 4th day of September, 2001.
[RITUSHKA NEGI]
OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANTS